Glass reinforced organic polymeric substances are widely used materials, many of them successfully competing with metals because of their exceptional mechanical properties and ease of processing. The glass in such products is a reinforcement rather than a simple filler because the glass-resin composite derives its strength from the filaments of glass, much like concrete is reinforced with metal, e.g., steel, so that the two materials act together in resisting forces.
It is essential to the achievement of maximum properties in glass-reinforced plastics that the resin, i.e., organic polymeric substance, wet the glass, i.e., there should be good bonding between such substance and the glass filaments. Most plastic substances do not bond well to glass, so it is an accepted practice to coat the glass filaments with an intermediate layer of a sizing substance which wets and adheres to the glass and which is compatible with the superjacent plastic substance.
Many glass reinforced plastics have excellent short-term as well as long-term temperature resistance. Most of them have high strength and toughness and some have remarkably good hydrolytic stability and excellent electrical properties. The increasing number of end uses--as metals are displaced--requires considerable attention to improving the fire retardancy of glass reinforced composite materials. Fire retardancy is a critical requirement in automotive and electrical uses and in the construction industry.
The main factor which determines the fire resistance of a glass-resin composite is the resin itself. There are wide variations in the burning rate among the common resins. Shellac and cellulose esters, for example, burn readily at comparatively low temperatures. Styrene resins, on the other hand, burn slowly and they drip. Still other resins such as vinyl resins, e.g., poly(vinyl halides); polyphenylene oxide; and polycarbonates, are self-extinguishing, i.e., they will melt or disintegrate without actually burning, or will burn only if exposed to direct flame. In most cases, however, it is necessary to compound certain chemicals into all resins used in glass-resin composites to reduce or eliminate the tendency of the composite to burn and drip. Such chemicals are known as flame-retardant additives, e.g., for polyethylene and similar resins, antimony trioxide and chlorinated paraffins are useful flame-retardant additives.
A primary, but obscure, factor involved in the tendency of glass-resin composite to burn, even if the resin is self-extinguishing per se, is the lack of fire resistance of the glass filamentous reinforcement. Of course, it is not likely that the glass filaments per se would promote burning of the composite material under most conditions, in spite of their very large surface area. However, the conventional sizing compounds, which cover the fine glass filaments have unexpectedly now been found to constitute a large flammable surface area, with the result that composites reinforced with conventionally sized filamentous glass contribute a vulnerability to flame attack out of all expected proportion to the quantities of sizing used. For example, a typical composite may contain 30 percent by weight of glass which is coated with 1.5 percent by weight of the glass of a combustible glass size, e.g., polyvinyl acetate or polyester. Such a composite, even though it contains only 0.4 percent by weight of sizing, cannot easily meet the requirements for classification as a self-extinguishing and non-dripping article according to ASTM test method D-635 and Underwriters' Laboratories Bulletin No. 94. This is true even if the resin per se in the composite is self-extinguishing and non-dripping. It is, therefore, necessary and conventional to load the composites with a relatively high amount of flame-retardant additives. Such composites with large amounts of additives, most of which plasticize the resin to some extent, suffer losses in physical properties, such as heat distortion temperature, and in electrical properties, and are expensive to produce.
It has now unexpectedly been found that glass-resin composites can be rendered flame-retardant by using as a reinforcement, filamentous glass which is coated with a non-burning or self-extinguishing polymeric sizing composition. This technique has two unexpected advantages, which are again seemingly out of proportion to the amount of sizing actually present in the composite: (a) it is possible to use lower amounts of flame-retardant additives with both flammable, e.g., polystyrene, and with non-burning and self-extinguishing, e.g., polycarbonate and polyphenylene oxide, resins superjacent the sized filaments, and (b) it is possible to prepare flame-retardant composites from normally non-burning or self-extinguishing resins, e.g., polyphenylene oxide resins, without the need to add flame-retardants to the resin.
For the purposes of this disclosure and the appended claims, the term "non-burning or self-extinguishing" contemplates properties measured by such tests as ASTM D-635 and that described in Underwriters' Bulletin No. 94. More particularly, in the latter test, a molded piece of about 21/2" by 1/2" by 1/8" is formed from the reinforcement and the resin and if it does not drip upon ignition and will extinguish itself within 30 seconds, after two 10-second ignitions, the composite is deemed to be flame-retardant to the point where it satisfies the requirements set forth by the Underwriters' Laboratories. Test ASTM D-635 for flammability comprises contacting the end of a specimen 1/2" by 5" and "thickness normally supplied" with a Bunsen burner flame for 30 seconds; and repeating if there is no ignition. If the specimen does not ignite, it is classified "non-burning by this test". If the specimen does ignite but does not continue burning to the 4" mark, after the flame is removed, it is classed as "self-extinguishing by this test."
It is, accordingly, a primary object of this invention to provide filamentous glass reinforcements for non-burning or self-extinguishing glass-resin composites.
It is a further object of this invention to provide molding powders suitable to prepare non-burning or self-extinguishing glass-resin composites in any desired form.
Still another object of this invention is to provide three dimensional reinforced glass-resin composite articles which are non-burning or self-extinguishing.
A further object of this invention is to provide glass-resin composites with excellent flame-retardant properties using lower amounts of flame-retardant additives than heretofore.
Still another object of the invention is to provide glass-resin composites with flame-retardant properties using self-extinguishing resins and no flame-retardant additives.